Modified Double Disc Synergy Test to Detect ESBL Production in Urinary Isolates of Escherichia coli and Klebsiella pneumoniae.

BACKGROUND AND OBJECTIVES Various phenotypic methods are recommended in the routine practice to detect the ESBL production in gram negative bacilli. Among them, the Double Disc Synergy Test (DDST) which uses the third generation cephalosporins (3GC), is a simple and a reliable method. But the coexistence of AmpC may give false negative results. In such cases, the ESBL detection can be improved by using cefepime along with the third generation cephalosporins in DDST. METHODS A total of 350 urinary isolates (224 Escherichia coli and 126 Klebsiella pneumoniae) were studied for ESBL production by the modified double disc test (MDDST) i.e. by using cefotaxime, ceftriaxone, cefpopdoxime (third generation cephalosporins) and cefepime ( fourth generation cephalosporin) along with a amoxicillin-clavulanate disc. RESULTS AND INTERPRETATION ESBL production was seen in 63.4% (142/224) Escherichia coli and in 60.3% (76/126) Klebsiella pneumoniae isolates by MDDST. Among these, in twelve E.coli and five K.pneumoniae strains, only cefepime but none of the third generation cephalosporins showed synergism with amoxicillin-clavulanate. All these seventeen strains showed a clear extension of the edge of inhibition which was produced by cefepime towards the amoxicillin-clavulanate disc. These strains were further tested for AmpC co-production by the AmpC disc test and all these strains were found to be AmpC positive, thus revealing the superior activity of cefepime in detecting ESBLs in the bacteria which co-produced AmpC. A high degree of coresistance was found in the ESBL producers. CONCLUSION The ESBL detection can be improved by MDDST by using cefepime along with the third generation cephalosporins.